With recent implementation of a blue diode in practical use, studies are being aggressively made to develop a white light-emitting diode by utilizing the blue diode as a light source. The white light-emitting diode is lightweight, uses no mercury and has a long life, and the demand therefore is expected to rapidly expand in the future. The white light-emitting diode usually employed is prepared by coating a blue light-emitting device with a paste of a mixture of cerium-activated YAG (Y3Al5O12:Ce) powder and epoxy resin (see, Japanese Unexamined Patent Publication (Kokai) No. 2000-208815).
However, the color of the YAG:Ce photoluminescence is present in the vicinity of x=0.41 and y=0.56 in the CE chromaticity coordinates and when mixed with the color of blue excited light at 460 nm, the color tone is controlled in a line connecting the color coordinates of the blue light-emitting diode and the color coordinates of YAG and therefore, the color is not white but becomes white mixed with green-blue color. Accordingly, there arises a problem that a white color insufficient in the red color results. In order to solve this problem of bad color tone, another phosphor powder of emitting a red color is mixed with the YAG:Ce phosphor powder, thereby controlling the color tone.
However, the number of reports on a phosphor of absorbing blue light and emitting red photoluminescence is very small. Specifically, for example, Ba2Si5N8 activated by europium (Eu) has been reported in Journal of Physics and Chemistry of Solids, Vol. 61, pp. 2001-2006 (2000). Also, in recent years, Eu-activated CaAlSiN3 having an emission intensity surpassing that of Ba2Si5N8 has been found (see, Extended abstracts, The 65th Autumn Meeting, 2004, page 1283, The Japan Society of Applied Physics.)
Under these circumstances, we investigated for a novel red phosphor. In particular, since it has been experimentally found that in CaAlSiN3:Eu, AlN is liable to remain, studies have been aggressively made on a composition system with less AlN. The AlN when Eu is contained therein emits green light in the vicinity of 535 nm and therefore, mingling of AlN into a red phosphor is not preferred. It could therefore be advantageous to provide a new red nitride phosphor capable of absorbing blue light and emitting red photoluminescence.